I’m assuming (since the OP died) that you mean “why does hair grow to a certain length, then grow again when cut?”
The answer is that hair only grows to a fixed length, then it falls out. If you never cut your hair, your hair will grow to that length, and when the old hairs get too long, they fall out, and are replaced by new hair. You don’t notice an overall difference because at any one time, you have tons of long hairs, and tons of shorter hairs. If you cut your hair often, all the hair is below that maximum length, and so all the hair will grow.
Thank you, the OP did indeed wander off into the virtual wasteland apparently.
Yup, that was pretty much the question. I know (for most of us) head hair grows as long as you allow it to, but what about leg,or arm hair? Or eyebrow hair? I realize that it’s genetically programmed to grow to a certain length, and is periodically replaced, but what signals that indivual hair that it has been trimmed?
In other words:
If I took an eyebrow hair, or an arm hair, and trimmed it this week, why, by next week (or so) is it back to it’s original length? What tells it that “it has been trimmed, grow this much, and stop”?
And I realize that Unca Cece did the column, but the specific question really was not answered. A general “this is how hair grows” answer was given instead.
I think Cecil answered your question completely. Nothing tells the hair that it has been trimmed. When you trim your arm hair, the hair that grows back to the same length as before is not the same strands that were cut. The strands that were cut will never reach full length, eventually they will fall out and be replaced by an uncut hair that will reach full lengther.